In conventional ignition coils for use in internal combustion engines, the inside of a plug hole communicates with an outside atmospheric pressure side via an air path disposed at a boundary between an outer periphery of the ignition coil and a plug hole seal. This is done to discharge/take air from/to the inside of the plug hole to facilitate removal of the ignition coil, the air being produced when the temperature inside the plug hole changes or the ignition coil is removed or reinstalled. A change in ambient temperature or the internal combustion engine being splashed with water, however, causes the temperature of the plug hole to be reduced, resulting in a change in pressure inside the plug hole. Thus, to achieve equilibrium with the outside air, a negative pressure is created to draw the outside air in. Then, the negative pressure may cause water to enter the plug hole from the outside via an opening in the air path on the atmospheric side.
A known arrangement includes a pool section (water retention chamber) for storing therein water that enters the air path (see, for example, Patent Document 1). Water entering from the outside is retained in the pool section and thus prevented from entering the plug hole.